Today marks not one but two milestones in planetary exploration. It is the 25th anniversary of Voyager 2’s flight past Neptune, the most distant planet ever seen up close. It is also the exact day that the New Horizons spacecraft is crossing Neptune’s orbit on its way to Pluto, the mysterious world that marks the boundary between the solar system we know and the one we don’t.

Flight path of New Horizons. Don’t look for Earth’s orbit–it’s too small to see. (Credit: NASA/JHUAPL)

The known solar system has planets that come in three well-studied varieties: rocky (like Earth), gas giant (like Jupiter), and ice giant (like Neptune). Beyond Neptune, things get complicated and confusing. There’s Pluto, but there’s also the whole Kuiper Belt, a vast collection of other, related objects. Most are the size of small planetary moons, but a few are roughly the size of Pluto and some, yet unseen, might be even bigger. Beyond that is a region called the “scattered disk,” where recurring comets come from. And beyond that comes the really shadowy territory: The Oort Cloud, an inferred swarm of dormant comets stretching almost halfway to the next star.

Voyager 2’s images of Neptune and its satellites are still stunning and largely under-appreciated. More fascinating even than Neptune itself is its giant moon Triton, which is similar in size and density to Pluto but has lived a very different life. Triton may have started out as Pluto’s near twin, but it got captured into a backward (clockwise) orbit around Neptune. Gravitational interaction between satellite and planet generate heat and keep Triton active. Its surface has few craters and looks geologically active. Triton offers a hint at what awaits in the Kuiper Belt. Pluto will show us for real what’s out there.

I’m boycotting the whole debate over whether Pluto is a planet or not, because it misses the point. The Kuiper Belt and scattered disk cover account for about 99.8 percent of the volume of the solar system (the Oort Cloud is so ridiculously large I won’t even get into it). We’ve seen only a few examples of the objects that wander sunward and become comets, and we’ve never seen any of them in their native environment. We are still strangers in our own solar system, and Pluto is the first step in getting to know the rest of the neighborhood.

Planet? Dwarf planet? I just call it damn interesting. So I’m looking back at some of the signature images from Voyager 2 at Neptune, and looking forward to what New Horizons will be seeing just 11 months from now.

1. Rings of Neptune. That’s right, Neptune has rings, but they’re thin and faint, nearly invisible from Earth. Here Voyager 2 looks at them backlit by the sun, with the planet severely overexposed by the two, combined 10-minute exposures. All images credited to NASA/JPL unless noted otherwise.

2. Clouds of Neptune. Despite its great distance from the sun, Neptune has dynamic weather, including the highest wind speeds recorded anywhere in the solar system. The “great dark spot” seen here has since disappeared, showing how quickly things change out there.

3. Triton in full. Paul Schenk at the Lunar and Planetary Institute reprocessed the old Voyager data to bring out more detail and correct colors. This is the result: the best-ever view of Triton’s two hemispheres, though much of the moon’s northern half is missing because it was in shadow at the time.

4. Geysers of Triton. This Voyager close-up contains two surprises. First, there are almost no craters, indicating that the surface is only about 10 million years old–very young in geological terms. Second, what’s up with those bizarre streaks? They appear to be eruptions of some sort, perhaps from gases warmed by the sun, that shot out clouds of fine dust; the extremely thin atmosphere then carried the particles downwind.

5. Neptune from Triton. By combining Voyager images with topographic measurements, NASA researchers put together this composite perspective view. The smooth plains suggest that Triton has been reshaped by icy volcanism.

6. Goodbye–for now. Three days after encounter, Voyager 2 looked back at the receding crescents of Neptune and Triton. Soon New Horizons will be on its way to pick up where Voyager left off, completing the solar system “grand tour” originally proposed by NASA in 1964 (!).

Long after we are gone, long after the earth is swallowed by the dying sun, mankind’s missionaries, the Voyagers, will pursue their cold and lonely arc through the Universe…Something of men will live after them.

Philip Benson

Whats the probability of a new technology spacecraft passing Voyager, and how soon could this happen?

Corey S. Powell

No currently planned mission will overtake Voyager 1–not even New Horizons–but I sincerely hope (and assume) that some future technology will allow much faster travel.

What you describe is a common theme in science fiction and in scientific speculation about interstellar travel. It is often called the “incessant obsolescence paradox”: the idea that early space explorers will constantly be overtaken by later explorers using newer, faster technology. More on that here:

AHH jk i know that the earth will be destroyed be … drum roll….. THE SUN!!!! D:

coreyspowell

Oh, it will happen. Might take a few billion years, but it will happen.

Bluestar

I know but AHHH anyway.

JerryG

I was an Asst. DSIF Project Engr for Pioneer 10 at JPL in 1971, then at that time, it was to be the 1st spacecraft to leave the solar system. The mission was changed later and Voyager received that distinction. In 1966 I was an Asst. DSIF Project Engr on the 1st spacecraft to orbit the moon, Lunar Orbiter. My point is we are close to a half of a century of deep space exploration, and we are still using the same technology we started with, Oh yes, it is definitely advanced, it did get bigger and better; but it is still the same, we are still playing with rockets and locked close to earth.

We do have the technology today to move away from rockets, and turtle speeds in moving through deep space. With a dedicated research and development concentration of a project to develop real teleportation, humanity can start to achieve its real destiny. With real teleporting, using current scientific technology (not all their thinking) both distance and time evaporates to nothing in traveling in deep space. Ask any UFO!

Understand it is scientific thinking that prohibits real teleportation today, not the science’s physics and technology. Review the five videos of a proposed project called Bmeup Teleport, that postulates we could be teleporting by the end of this decade. http://BmeupTeleport.com

Bluestar

The sun says,” Yummy, planets are really good tasty.”

Bonusje

Moon mo-on emo – on means belonging to or a satellite like bankimoon means satellite of the banks an agent of the centralbanksters who leads the united nations ie the World as commonwealths of China setup by the chinese ie sun yat sen1905 communist the stalin of china and first president of china after slaughtering some 80 million Chinese Who Wanted an Other Life instead of stinking industries and endless debts. Same the chinese did at same time to us also.

But anyway united nations and bankimoon are satellites of china.

As yat sen yuk is a donkey of China .. ie the centralbanksters .. a emmisary.

Planet means pfa la en et der grosse chinese around which the World turns pfa alah en et – to Alah it Belongs .. chinese Chi enes genes genesis .. emanation of GOD. Other explanation .. Plane et in the Plane of the disk of the other Planets .. or At GOD`s WILL.

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About Corey S. Powell

Corey S. Powell is DISCOVER's Editor at Large and former Editor in Chief. Previously he has sat on the board of editors of Scientific American, taught science journalism at NYU, and been fired from NASA. Corey is the author of "20 Ways the World Could End," one of the first doomsday manuals, and "God in the Equation," an examination of the spiritual impulse in modern cosmology. He lives in Brooklyn, under nearly starless skies.